1. Field of the Invention
The present invention relates to an emissive display device using emissive elements, such as electroluminescence elements, which employs thin film transistors for controlling such elements.
2. Description of the Related Art
In recent years, electroluminescence (referred to hereinafter as “EL”) display devices comprising EL elements have gained attention as potential replacements for CRTs and LCDs. Research has been directed to the development of EL display devices using, for example, thin film transistors (referred to hereinafter as “TFT”) as switching elements to drive the EL elements.
FIG. 1 is a plan view showing one display pixel of an organic EL display device. FIG. 2 illustrates an equivalent circuit for a plurality of display pixels in an organic EL display device. FIG. 3A shows a cross-sectional view taken along line A—A of FIG. 1, while FIG. 3B shows a cross-sectional view taken along line B—B of FIG. 1.
As shown in FIGS. 1, 2, 3A, and 3B, each display pixel is formed in a region surrounded by gate signal lines 151 and drain signal lines 152. A first TFT serving as a switching element is disposed near a intersection of those signal lines. The source 131s of the TFT simultaneously functions as a capacitor electrode 155 such that, together with the opposing storage capacitor electrode 154 described later, it forms a capacitor. The source 131s is connected to a gate electrode 142 of a second TFT 140 that drives the organic EL element. The source 141s of the second TFT 140 contacts with the anode 161 of the organic EL element. The drain 141d is connected to a power source line 153.
Near the TFT 130, a storage capacitor electrode 154 is disposed in parallel with a gate signal line 151. The storage capacitor electrode 154 is made of a material such as chromium. The storage capacitor electrode 154 contacts the capacitor electrode 155 via a gate insulating film 112 and together stores charges, forming a capacitor. The capacitor electrode 155 is connected to the source 131s of the first TFT 130. This storage capacitor is provided for retaining voltage applied to the gate 142 of the second TFT 140.
The first TFT 130, or the switching TFT, will now be explained.
As shown in FIGS. 1 and 3A, gate signal lines 151 made of refractory metal such as chromium (Cr) or molybdenum (Mo), which also serve as gate electrodes 132, are formed on an insulator substrate 110 made of quartz glass, non-alkali glass, or a similar material. Also disposed on the substrate 110 are drain signal lines 152 composed of aluminum (Al) and power source lines 153 also composed of Al and serving as the power source for the organic EL elements.
After forming gate signal lines 151 on the substrate 110, a gate insulating film 112 and an active layer 131 composed of poly-silicon (referred to hereinafter as “p-Si”) film are sequentially formed. The active layer 131 is of a so-called LDD (Lightly Doped Drain) structure. Specifically, low-concentration regions 131LD are formed on both sides of each gate 132. The source 131s and the drain 131d, which are high-concentration regions, are further disposed on the outboard sides of the low-concentration regions 131LD.
An interlayer insulating film 115 formed by sequential lamination of a SiO2 film, a SiN film, and a SiO2 film is provided on the entire surface over the gate insulating film 112, the active layer 131, and stopper insulating films 114. A contact hole formed in a position corresponding to the drain 141d is filled with metal such as Al, forming a drain electrode 116. Further, a planarizing insulating film 117 made of an organic resin or a similar material is formed over the entire surface for planarization.
The second TFT 140, or the TFT for driving the organic EL element, will next be described.
As shown in FIG. 3B, gate electrodes 142 composed of refractory metal such as Cr or Mo are formed on the insulator substrate 110 made of quartz glass, non-alkali glass, or a similar material. Further on top, a gate insulating film 112 and an active layer 141 composed of p-Si film are sequentially formed. The active layer 141 is provided with intrinsic or substantially intrinsic channels 141c formed above the gate electrodes 142, and the source 141s and drain 141d are formed on respective sides of these channels 141c by ion doping using p-type impurities, thereby constituting a p-type channel TFT.
An interlayer insulating film 115 formed by sequential lamination of a SiO2 film, a SiN film, and a SiO2 film is provided on the entire surface over the gate insulating film 112 and the active layer 141. A contact hole formed in a position corresponding to the drain 141d is filled with metal such as Al, forming a power source line 153 connected to a power source. Further, a planarizing insulating film 117 made of an organic resin or a similar material is formed over the entire surface for planarization. A contact hole is formed in the planarizing insulating film 117 in a position corresponding to the source 141s. A transparent electrode made of ITO (indium tin oxide) that contacts the source 141s through this contact hole, namely, the anode 161 of the organic EL element, is formed on the planarizing insulating film 117.
The organic EL element 160 is formed by laminating, in order, the anode 161 constituted by a transparent electrode made of ITO or similar material, an emissive element layer 166 which is composed with materials including an organic compound and comprises an emissive layer, and a cathode 167 made of a magnesium-indium alloy. The cathode 167 is disposed over the entire surface of the organic EL display element shown in FIG. 1, that is covering the entire sheet of the figure.
In an organic EL element, holes injected from the anode and electrons injected from the cathode recombine in the emissive layer. As a result, organic molecules constituting the emissive layer are excited, generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode via the transparent insulator substrate, resulting in light emission.
In this way, electric charge applied via the source 131s of the first TFT 130 is accumulated in the storage capacitor 170 and applied to the gate 142 of the second TFT 140. According to this voltage, the organic EL element emits light.
As shown in FIG. 2, each power source line connected to the power source for driving the organic EL elements is connected with a power source input terminal 180 disposed outside the display pixel region. The power source lines are arranged and connected with each vertical array of display pixels. With such an arrangement, at positions more distant from the power source input terminal 180 resistance of each power source line increases along with its length. The organic EL elements 160 located in display pixels distant from the power source input terminal 180 are therefore not adequately provided with necessary current, causing a disadvantage that the display in such area is dim.